<!DOCTYPE html>
<html lang="en">
<head>
    <style>
        .card
        {
            border: 1px solid #cccccc;
            background: #f4f4f4;
            padding: 20px;
            margin-bottom: auto;
        }
    </style>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>ID and Class</title>
</head>
<body>
    <div id="main-header" class="card">
        <h1>我的网站</h1>
        <p>主要是有关xxx的内容</p>
    </div>
    <ul id="main-nav">
        <li><a href="#">主页</a></li>
        <li><a href="#">关于</a></li>
        <li><a href="#">联系</a></li>
    </ul>

    <div id="about">
        <h3>关于我们</h3>
        Lorem ipsum dolor sit, amet consectetur adipisicing elit. Ullam aut sed quos, nobis corrupti aperiam. Repellat amet ullam eligendi nisi?
        
    </div>

    <div id="contact">
        <h3>联系我们</h3>

    </div>

    <div id="footer">
        <p>Copyright 2020</p>
    </div>
</body>
</html>